It is now commonplace to locate computers, keyboards, and monitors, particularly color monitors, at spaced locations in a building or buildings. These locations often are several hundred feet apart, requiring that where analog color signals are involved that there must be transmitted three separate color signals, each having an approximate frequency range from D.C. up to 200 MHZ. Thus, there is a requirement that appropriate transmission lines be in place, or be installed, to accommodate such transmissions. As is well known, either fiber optic or multiple coaxial cables may normally be employed, but such is often not available. Thus, there may be required by an occupant of a building that appropriate signal conductors be after fitted to the building. This can result in a considerable cost. Ideally, there would be present, or there might be installed at a relatively low cost, lower-grade conductors, such as network cable or twisted pair cable and that it be somehow used.
In a co-pending application, application Ser. No. 08/177,442, the existing cable was of the digital network type, for example, having 15 conductors within an outer shield and designed to carry on the order of 2,400 baud rate signals and wherein there existed straight (untwisted) conductors.
The problem in that case was to overcome frequency deficiencies and to overcome interaction between colors as finally received. The solution was that of discovering appropriate frequency-amplitude compensation plus effecting a phase reversal of one color signal appearing on one conductor (with respect to shield) and positioning this conductor between conductors carrying the other two color signals. At the receiver, the phase reversal was reversed back.
The present invention deals with a second type of cable, basically telephone (voice frequency) cable wherein there is included a plurality of twisted pair-type conductors, typically four pairs for the carrying of as many communications.
It too has unique problems with respect to frequency compensation. A second problem appears from the finding that different sets of twisted pairs, and in different cables, have a variety of twist rates, different twist rates for a given cable being provided to prevent telephone cross-talk between communications on different twisted pairs of a cable. Unfortunately, the applicants have found that the latter was a culprit in preventing good color signal transmissions since a composite of three color signals, sent on separate twisted pairs, is required, and the different twist rates of conductor pairs caused the lengths of the pairs and signal delays to differ. This in turn resulted in the receipt of a composite of color signals with observable impurities and thus an unsatisfactory presentation on a color monitor.
Significant, however, was the substantial availability of such cable and that it is already installed in many buildings where color transmissions were now needed. Thus, if it could be employed, such would enable a tremendous saving, a mark of clear technical achievement in view of the fact that the problem has remained unsolved for at least 10 years.
The applicants have discovered that relatively high frequency color video signals may be transmitted with high color purity over a cable having multiple, relatively low frequency, twisted pair telephone lines and despite their having different twist rates, which rates are non-uniform as between cable manufacturers. The applicants have solved the problem by effecting certain selected frequency compensation to color signals at each end of a cable and by discretely applying delays to the two twisted pair lines having lower twist rates. Alternately, in certain instances, applicants have discovered that adequate color purity can be achievable over cable runs of 300 feet or less by connecting the red video signals to the twisted pairs having the smallest twist rate (i.e., lowest twist rate), the green video signals to the twisted pair having the next highest twist rate, and the blue video signals through the twisted pair having the third largest twist rate. Typically, then, the synchronization signals would be connected through the cable having the largest twist rate (or tightest twist rate), which is not as critical.